The present disclosure relates to a method of forming a semiconductor structure, and more particularly to a method of forming a semiconductor structure which includes an organic light emitting diode (OLED) display with OLED driving circuitry, logic and memory devices, as well as other functions integrated on a same single crystalline semiconductor substrate. The present disclosure also relates to the semiconductor structure that is formed utilizing the method of the present disclosure.
In recent years, mobile information and telecommunication devices such as computing Notebooks and personal digital assistants (PDAs) have experienced rapid development. Such devices are becoming lighter and more efficient. Recently, flat panel displays are becoming more and more popular for such devices. Currently, liquid crystal displays (LCDs) are used as flat panel displays, although LCDs have some disadvantages, e.g., the need for background lighting and a limited viewing angle.
Besides liquid crystals, organic light emitting diodes, so-called “organic LEDs”, or “OLEDs”, can be used in flat panel displays. Such OLEDs have a higher luminous efficiency and an increased viewing angle as compared to LCD's. The basic feature of the OLED is the electroluminescence of specific organic materials. The specific organic material determines in a first approximation the color, i.e., the wavelengths, of the light emitted by the corresponding OLED.
A typical OLED comprises a substrate which is usually made of glass or a similar transparent material. An anode layer is positioned on the substrate. The anode layer can be made of a material having a relatively high work function and is substantially transparent for visible light. A typical material for the anode layer is indium tin oxide (ITO). A layer of electroluminescent material is positioned on the anode layer, serving as the emitting layer of the organic OLED. Common materials for forming the emitting layer are polymers such as, for example, poly(p-phenylenvinylene) (PPV) and molecules like tris(8-oxychinolinato)aluminum (Alq3). In the case of molecules, the emitting layer typically comprises several layers of the molecules. A cathode layer of material having a lower work function like aluminum (Al), calcium (Ca) or magnesium (Mg) is positioned on the emitting layer. During operation of the OLED, the cathode layer and the anode layer are connected to a power supply.
The basic principles of electroluminescence and, thus, of the OLED are the following: The anode layer and the cathode layer inject charge carriers, i.e., electrons and holes, into the emitting layer. In the emitting layer, the charge carriers are transported and the charge carriers of opposite charge form so called excitons, i.e., excited states. The excitons decay radiatively into the ground state by generating light. The generated light is then emitted by the OLED through the anode layer which is made of transparent material like ITO. The color of the generated light depends on the material used for the organic layer.
Furthermore, a so-called multilayer OLED is known. The multilayer OLED comprises a plurality of cathode layers and/or a plurality of organic layers and/or a plurality of anode layers. By using a plurality of organic layers, the efficiency of the OLED can be increased compared to the organic LED comprising a single organic layer. The boundary surface between two organic layers of the plurality of organic layers can act as a barrier which reduces the current flow through the diode for at least one charge carrier type (electrons or holes). Therefore, the at least one charge carrier type accumulates at the boundary surface and thus the recombination probability of the electrons and the holes is increased leading to a higher efficiency of the OLED.